US Marines of the 1st Division pass by a wall painting of an Iraqi Army soldier during training in a former Iraqi army barrack outside Fallujah, Iraq, Thursday, Nov. 4, 2004. U.S. forces are preparing for a major offensive in Fallujah, west of Baghdad, and other Sunni militant strongholds in hopes of curbing the insurgency ahead of January's election. (AP Photo/Anja Niedringhaus) less

US Marines of the 1st Division pass by a wall painting of an Iraqi Army soldier during training in a former Iraqi army barrack outside Fallujah, Iraq, Thursday, Nov. 4, 2004. U.S. forces are preparing for a ... more

Photo: ANJA NIEDRINGHAUS

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A legend for a city map of Fallujah, made out of gravels and Lego stones, is on display for training purpose at a base outside Fallujah, Iraq, Friday, Nov. 5 , 2004. More than 10,000 U.S. troops have taken positions around rebel-controlled Fallujah, bolstering the U.S. Marine units expected to lead a joint Army-Marine assault on the city. (AP Photo/Anja Niedringhaus) less

A legend for a city map of Fallujah, made out of gravels and Lego stones, is on display for training purpose at a base outside Fallujah, Iraq, Friday, Nov. 5 , 2004. More than 10,000 U.S. troops have taken ... more

Photo: ANJA NIEDRINGHAUS

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US Marines of the 1st Division prepare their vehicles at a base outside Fallujah, Iraq, Friday, Nov. 5, 2004. More than 10,000 U.S. troops have taken positions around the rebel-controlled city of Fallujah, bolstering the U.S. Marine units expected to lead a joint Army-Marine assaul. (AP Photo/Anja Niedringhaus) less

US Marines of the 1st Division prepare their vehicles at a base outside Fallujah, Iraq, Friday, Nov. 5, 2004. More than 10,000 U.S. troops have taken positions around the rebel-controlled city of Fallujah, ... more

Photo: ANJA NIEDRINGHAUS

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A US Marine of the 1st Division checks ammunition at a base outside Fallujah, Iraq, Friday, Nov. 5 , 2004. More than 10,000 U.S. troops have taken positions around the rebel-controlled city of Fallujah, bolstering the U.S. Marine units expected to lead a joint Army-Marine assault on the city. (AP Photo/Anja Niedringhaus) less

A US Marine of the 1st Division checks ammunition at a base outside Fallujah, Iraq, Friday, Nov. 5 , 2004. More than 10,000 U.S. troops have taken positions around the rebel-controlled city of Fallujah, ... more

Photo: ANJA NIEDRINGHAUS

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A US Marine of the 1st Division listens to music while cleaning his gun at a base outside Fallujah, Iraq, Friday, Nov. 5 , 2004. More than 10,000 U.S. troops have taken positions around the rebel-controlled city of Fallujah, bolstering the U.S. Marine units expected to lead a joint Army-Marine assault on the city. (AP Photo/Anja Niedringhaus) less

A US Marine of the 1st Division listens to music while cleaning his gun at a base outside Fallujah, Iraq, Friday, Nov. 5 , 2004. More than 10,000 U.S. troops have taken positions around the rebel-controlled ... more

2004-11-06 04:00:00 PDT Near Fallujah, Iraq -- U.S. armored vehicles roared through the villages surrounding Fallujah, the town at the heart of the Sunni insurgency in Iraq, as warplanes pounded rebel positions and ground forces ratcheted up their preparations Friday for what appeared to be an imminent assault on the city.

Within Fallujah, insurgents who were hiding themselves by day among a dwindling and embittered populace set up a defensive perimeter around the city and said they would defeat the Americans or die in a cause they called just.

Marines gathering outside the city practiced house-to-house fighting, while some American crews fitted their armored vehicles with front-loading shovels designed to unearth explosives buried in the roads on the way in. Marines fired artillery rounds throughout the day and night on positions around the city.

"We are going to rid the city of insurgents," said Lt. Col. Gary Brandl, a battalion commander in charge of about 800 Marines at a base outside the city. "If they do fight, we will kill them."

Military intelligence officials say that as many as 75 to 80 percent of the city's 250,000 residents have fled. That estimate is consistent with reports from inside Fallujah.

As battle preparations went forward, top U.S. commanders in Iraq and senior Bush administration officials in Washington were conducting final reviews of their own.

U.S. officials said that the precise timing of any operation was being left to U.S. commanders in the field and to Iraqi Prime Minister Ayad Allawi.

Visiting European Union leaders in Brussels on Friday, Allawi reiterated his warning that "the window is really closing" on the chances for a peaceful settlement of the standoff. Negotiators for the two sides have not met in more than a week.

Meanwhile, at the United Nations, Secretary-General Kofi Annan confirmed that he has formally expressed concern about the effects any invasion of Fallujah would have on stability in the country ahead of elections scheduled for January.

In letters dated Oct. 31 and addressed to President Bush, British Prime Minister Tony Blair and Allawi, Annan said that using military force against insurgents in the city would further alienate Sunni Muslims already feeling left out of a political process largely orchestrated by Washington.

"I don't know what pressure he has to bear on the insurgents," Allawi said in an interview with the BBC. "If he can stop the insurgents from inflicting damage and killing the Iraqis, then he's welcome, we will do whatever he wants."

Allawi and U.S. officials have insisted that they must reassert control over Fallujah quickly in order to pave the way for the elections. Fallujah lies squarely within a region of the country dominated by Sunni Arabs, a minority group whose participation in the elections is considered crucial if the outcome is to be accepted as legitimate. Favored under Saddam Hussein's rule, disenfranchised Sunnis are now leading the increasingly deadly insurgency.

Outside the city, the Americans were setting up military checkpoints to choke off access roads. Warplanes conducted at least five major air strikes on Friday.

Inside Fallujah, the insurgents appear to have the benefit of some fairly sophisticated military advice. They have built a layered perimeter with at least one inner fortified ring that would give them a place to retreat to should the outer ring be breached.

U.S. commanders in Iraq have expressed confidence they could complete their assault in a matter of days, but one senior officer said Friday that planners had no sure way of knowing how long insurgents would hold out. "Right now, they're hoping it doesn't go much longer than a week," the officer said.

Meanwhile, the insurgents continued with their deadly attacks. A U.S. soldier was killed and five were wounded in an attack on a base near Fallujah on Friday, the U.S. military reported. The injuries were "the result of an indirect fire attack," a term the military generally reserves for mortars or rockets, at about 1:45 p.m.

Two Marines were killed during security operations around Ramadi, just west of Fallujah, on Thursday, while one soldier in the 1st Infantry Division died and another was wounded in the town of Balad when an improvised bomb exploded near their vehicle. Balad is about 50 miles north of Baghdad on the road to Tikrit, Hussein's ancestral hometown.

The Fallujah operation is shaping up to be the largest since the U.S. invasion of the country 20 months ago. A senior military officer said that roughly 25,000 U.S. and Iraqi troops were surrounding Fallujah and Ramadi and the corridor between the two cities. Another senior military official said that 10,000 to 15,000 of those troops were immediately around Fallujah. They face an Iraqi insurgent force in the city that Brandl estimated at a few thousand fighters.

Iraqis likely to take control

Thousands of Iraqi troops have moved into position with their American counterparts and are expected to take part in the operation. According to the pattern set in similar operations in Najaf and Samarra, U.S. soldiers will do most of the fighting on the way in, clearing the way for the Iraqi security forces to take control of the city once the insurgents have been defeated. By this method, Iraqi and U.S. leaders are hoping that the operation in Fallujah will offer the best of both worlds: American muscle and an Iraqi face.

The performance of the Iraqi security forces is being viewed as particularly crucial in assessing the success or failure of the mission in Fallujah. In April, entire units of Iraqi police and national guard disintegrated before insurgents uprisings in Fallujah and southern Iraq. This time around, U.S. commanders say they have much higher hopes, particularly because of the intensive training Iraqi units have received.